Various types of screen are available on the market that are suited to be used in the sector of solid residue separation and are produced in different construction forms that operate according to different principles.
A known type of screen is constituted by the so-called “mesh screens” that comprise a supporting structure provided with a grid, along which the residues to be screened advance owing to the action of suitable moving means.
The grid consists of meshes with increasing cross section along the direction of advance of the residues, which are thus screened due to gravity during the advance, starting from the smaller-sized residues that fall under the grid in its initial section and proceeding until the larger-sized residues fall down at the level of the final section.
The screens of this type, however, pose a first recognized drawback represented by the fact that the grid wears out over time as the residues scrape against it and therefore must be replaced from time to time.
“Disc screens” are also known which comprise a supporting structure for a plurality of discs mounted on parallel rotary shafts.
The discs mounted on each rotary shaft are spaced from each other by a distance exceeding the thickness of each disc and therefore each disc of any of the rotary shafts is included between two discs of the adjacent shaft.
A screening surface is thus defined, in which the size of the openings created between the discs and the rotary shafts determines the degree of screening of the residues whose advance along the screening surface is determined by the rotation of the discs.
A disc screen of the type mentioned above is described in U.S. Pat. No. 4,972,959 and comprises a supporting structure of a series of parallel spaced shafts rotating in the same direction and supporting a plurality of discs. Also in this case a screening surface is defined, in which the rotation of the discs causes the advance of the residues that are thus screened according to the size of the openings defined between the discs and the shafts and fall under the screening surface due to gravity.
As each disc is coupled with the corresponding shaft through the interposition of elastic spacer sleeves, the discs may bend and deviate from their rotation plane if during screening any foreign matter should get stuck between them.
A disc screen is described also in Patent DE-U-297 11 724 and comprises a supporting structure associated with a plurality of rotating rollers arranged with their rotation axes parallel to each other and spaced by increasing distances according to the advance direction of the residues.
The rollers are connected to a plurality of discs arranged at a constant distance from each other, thus defining also in this case a screening surface constituted by the interspaces existing between the rollers and the discs.
All the rollers and therefore also the discs associated with them are set rotating at the same time and in the same direction so that the residue to be screened that is placed in the initial area is pushed forward by the rotation of the discs.
In this way, as the residues advance they are screened due to gravity, in fact the smaller-sized parts fall in the space between the discs.
Also the “disc screens” described above, however, pose some recognized drawbacks, the main of which lies in that their operation is easily affected or even prevented when among the residues to be screened there are filiform elements like rags, plastic bags, radio and video tapes, and similar elements.
In fact, this type of residues with mainly longitudinal development, which is frequently found among the residues when these are constituted by solid urban waste, tends to get twisted around the rollers and the shafts and to clog the spaces between the discs, thus affecting the screening operation and causing machine stoppages.
The user is thus forced to carry out frequent cleaning operations that are rather expensive because, among other things, they must be performed with the system at rest.
For these reasons “disc screens” have met with little success in the sector of solid residue screening.
In the attempt to overcome these drawbacks, the applicant of the present invention has registered in its own name the European Patent EP 1 106 264 that describes a screen for separating solid urban waste of the type with discs, provided with anti-clogging sleeves, useful above all when among the residues there are filiform elements.
According to the above mentioned patent, the screen comprises a supporting structure for a plurality of parallel and spaced rotary shafts, each one of which is provided with a plurality of discs, axially spaced from each other, between which a plurality of the so-called anti-clogging sleeves is provided.
With reference to FIG. 6, which illustrates a detail of the screen that is the subject of the above mentioned patent, between each disc indicated by A and the disc adjacent to it, not represented, there is an anti-clogging element indicated as a whole by B.
As can be observed, the anti-clogging sleeve comprises a cylindrical tube C externally coupled with the corresponding shaft D and having its inner diameter Ci that is longer than the outer diameter De of the shaft D and the length E shorter than the distance between the discs A.
Each cylindrical tube C is thus coupled with the corresponding shaft D with a play F and can rotate freely and translate radially in relation to it, both when the shaft D is at rest and when it is rotating.
In this way, if filiform residues get twisted around one or more cylindrical tubes C and increase their diameters until they come into contact with the opposing discs A, this does not affect the operation of the screen in any way and, most important of all, does not stop it.
In fact, each cylindrical tube C, being idle in relation to the shaft D with which it is coupled, is set rotating due to the friction that the residue twisted around it exerts against the discs A, substantially without absorbing power and therefore without interfering with the rotation of the shafts D.
Furthermore, the presence of said anti-clogging sleeves free to move between the discs favours the work of the operator when he/she has to intervene in order to remove any twisted filiform element from around the shafts and discs. Said sleeves, however, serve only partially the preventive anti-clogging function for which they are intended, that is, the function of limiting or in any case preventing the twisting of said filiform elements.
In fact, repeated observations have made it possible to find out that during the operation of the screen the anti-clogging sleeves are easily set rotating by the filiform elements present in the residues and this favours the twisting of said filiform elements.